Shell and tube heat exchangers



April 7,1970 R. G. PEARCE 3,504,739

SHELL AND TUBE HEAT EXCHANGEHS Filed June 6. 1968 2 Sheets-Sheet 1Inventor ROY PEARCE y yz %m Altorne April 7, 1970 PEARE 3,504,739

SHELL AND TUBE HEAT EXCHANGERS Filed June 6, 1968 2 Sheets-Sheet 2 Invenlor ROY 6 PEARCE Mom United States Patent O Int. Cl. Fzsf 9/02, 9/20US. Cl. 165134 12 Claims ABSTRACT OF THE DISCLOSURE This inventionrelates to improvements in shell and tube heat exchangers in which heatexchange takes place between one fluid passing through the tubes andanother fluid surrounding the tubes and within the shell. This heatexchanger has a tube plate which extends across the shell and to whichthe tubes are connected, and this tube plate is shielded by afluid-cooled shield plate, preferably liquid cooled, which is positionedin front of the tube plate and has stub tubes extending through it tofeed the hot fluid directly to the tubes of the heat exchanger, thearrangement being such that the opposite faces of the shield plate aresubjected to substantially identical pressures.

BACKGROUND OF THE INVENTION A typical shell and tube heat exchangercomprises a shell through which extend a large number of tubes in adirection parallel to the longitudinal axis of the shell. The ends ofthe tubes terminate in tube plates which divide the shell into end boxesto one of which fluid is fed before passage through the tubes and fromthe other of which the fluid is removed after passage through the tubesand a main heat exchange region containing the other fluid and throughwhich the tubes extend.

When the differential pressures and temperatures of the two fluids arenot excessive such a heat exchanger is perfectly satisfactory. When thisis not the case, however, serious problems can arise. If, for example,there is a large difference in pressure between the two fluids a thicktube plate is required and if the fluid fed through the tubes is verymuch hotter than the other fluid, this thick tube plate is subjected toa large temperature differential, and also it is in contact with veryhot fluid.

This situation frequently occurs in chemical plants Where heat recoveryis required from high temperature gases, e.g. 2,300 F., by using a fluidat, say, 500 F. In such a case it is essential that the tube plates beprotected from the fierce hot gases. This has been done in the past bycovering the surface of the plate exposed to the hot gases with a layerof refractory material and having ferrules or short tubes insertedthrough the layer of refractory material to provide access for the hotgases to the tubes. This solution still presents problems, however,because the ferrules or short tubes are liable to burn away at the hightemperatures to which they are exposed.

The invention has been made with these problems in mind.

According to one aspect of the invention there is provided a shell andtube heat exchanger in which the heat exchange tubes extend from a tubeplate which is subjected to hot inlet fluid, this tube plate beingshielded by a fluid cooled shield plate having flow means therethroughfor the passage of the hot inlet fluid to the tubes, the opposite facesofthe shield plate being in communication so that they are subjected tosubstantially identical pressures.

According to another aspect of the invention there is provided a shelland tube heat exchanger comprising a shell within which a number of heatexchange tubes 3,504,739 Patented Apr. 7, 1970 extending between tubeplates which define with the shell inlet and outlet end boxes for thefluid to pass through the tubes and a heat exchange region through whichthe tubes extend, a fluid-cooled shield plate spaced in front of thetube plate defining the inlet end box in the direction of flow of thefluid so as to shield that tube plate from very hot inlet fluids, andflow means extending through the shield plate for the passage of theinlet fluid from the inlet end box to the heat exchange tubes, theopposite faces of the shield plate being subjected to substantiallyidentical pressures, whereby the tube plate defining the inlet end boxis subjected to differential pressure but shielded by the shield platefrom a large differential temperature, which the shield plate issubjected to the inlet fluid, is cooled by the cooling fluid and is notsubjected to any substantial differential pressure.

The shield plate protects the surface of the tube plate from the veryhot fluid and so the tube plate can be thick so as to withstand thedifferential pressure without being subjected to a large temperaturedifferential. On the other hand the shield plate can be thin because ithas no differential pressure to withstand and since it is cooled,preferably by a liquid, no part of it reaches an excessively hightemperature or is subjected to a large temperature differential.

In general, the shield plate will be cooled by means of water and whenthis is the case an auxiliary supply of steam can be obtained.

DESCRIPTION OF THE DRAWINGS An embodiment of a heat exchanger accordingto the invention will now be described by way of example, with referenceto the accompanying diagrammatic drawings, in which:

FIGURE 1 is a longitudinal section through the heat exchanger; and

FIGURE 2 is a cross-section of part of the heat exchanger.

DESCRIPTION OF THE PREFERRED EMBODIMENT The heat exchanger 10 shown inthe drawings comprises a cylindrical shell 12 through which a largenumber of tubes 14 extend between a pair of tube plates 16. The tubesare fixed to the plates 16 in any convenient way, for example, bywelding or rolling.

The tube plates 16 define with the shell 12 a heat exchange region 18and an inlet end box 20 and an outlet end box 21. A very hot stream ofgases is fed to the inlet end box 20 shown and this passes through thetubes 14 and gives up heat to fluid, for example water, flowing throughthe region 18.

Positioned in front of the tube plate 16 and exposed to the very hotgases is a shield plate 22. This shield plate is spaced a short distancefrom the plate 16 and it comprises a pair of thin spaced circular plates24 which define between thzmselves a cooling chamber 26 which iscompleted by a band 28 to which both plates 24 are fixed. An annularring 29 engages the side of the band 28. Short stub tubes 30 extendright through both plates 24 and just into the ends of the tubes 14 soas to provide passages for the passage of the hot gases from the box 20to the tubes 14. The stub tubes 30 are a loose fit in the ends of thetubes 14 so that the pressure in the gap 32 between the tubes 14 and theshield plate 22 is substantially the same as that in the end box 20. Inthis way the shield plate 22 is not subjected to a differential pressureand it can therefore be made from thin material.

The cooling fluid for the shield plate 22, which is preferably water, issupplied to and removed from the shield plate through conduits 34, onlyone of which is shown.

The conduit 34 terminates at a flange 36 which is in turn connected withanother flange 38 on the end of a short tube 40 which passes to theoutside of the shell 12. This tube 40 passes through a layer ofrefractory material 42 whose purpose is to protect the inner wall of theshell 12. The band 28 and ring 29 are loosely fitted in the end box 20.This provides another means of preventing a large pressure differentialacross the shield plate 22.

The plate 22 is positioned by being pushed into place so that it is aloose fit within the refractory material 42. The flanges 36 and 38 arethen connected so as to hold the plate in place and complete the coolingfluid circuit. This simple fitting arrangement enables the shield plate22 to be removed and replaced or repaired if it ever becomes damaged bythe hot gases.

The flanges 36 and 38 are shown buried in the refractory material, thisensures that they are not burnt away. When the shield plate 22 is to befitted or removed the refractory material can be broken away and locallyrepaired after fitting the plate 22.

Because the plates 24 and stub tubes 30 are thin they can be keptreasonably cool by means of the cooling fluid although the plate 22 isexposed to very hot gases. At the same time the tube plate 16 isshielded from the hot gases and even if some of the hot gases pass intothe gap 32 either around the edges of the plate 22 or via the tubes 30they are quickly cooled by contact with the rear plate 24. Therefore nolarge temperature differentials are set up across the tube plate 14which must of necessity be quite thick to withstand differentialpressure between the fluids in the end boxes 20 and 21 and the region18.

The gap 32 between the plates 16 and 22 can be left empty as shown or ifdesired it can contain a layer of insulating or refractory material.

The arrangement shown for fixing the shield plate 22 in the shell is notessential. For example, the plate 22 can be attached to the innerperiphery of the shell by means of a flexible sealing device.

The cooling fluid, can be passed through the cooling chamber 26 byforced or natural circulation and in order to ensure an evendistribution of this fluid throughout the chamber 26 and thus for evencooling over the whole area of the plates 24 passages or baflles 40 maybe positioned in the chamber 26. Preferably the cooling fluid is waterand then a useful auxiliary supply of steam can be obtained and if thefluid being heated in the region 18 is water, the cooling water for theplate 22 can be derived from that supplied to the region 18 and theheated water or steam mixed with that heated in the region 18.

A latitude of modification, change and substitution is intended in theforegoing disclosure and in some instances some features of theinvention will be employed without a corresponding use of otherfeatures. Accordingly it is appropriate that the appended claims beconstrued broadly and in a manner consistent with the spirit and scopeof the invention herein.

I claim:

1. A shell and tube heat exchanger comprising a shell, heat exchangetubes, a tube plate from which said tubes extend, means for feeding ahot fluid to said tubes through said tube plate, a shell plate shieldingsaid tube plate, means for fluid cooling said shield plate, flow meansfor the passage of said hot fluid to said tubes through said shieldplate, and means for placing opposite faces of said shield plate in flowcommunication so that said opposite faces are subjected to substantiallyidentical pressures.

2. A heat exchanger according to claim 1 in which said shield plate isspaced a short distance in front of said tube plate in the direction offlow of said hot fluid, and said flow means direct said hot fluid intothe ends of said tubes.

3. A heat exchanger according to claim 1 in which said shield platecomprises a pair of spaced plates defining between themselves a chamber,and an inlet to and an outlet from said chamber for the passage ofcooling fluid through said chamber to cool said plate.

4. A heat exchanger according to claim 3 in which baflles are providedin said chamber.

5. A heat exchanger according to claim 2 in which said flow meansthrough said shield plate comprise stub tubes extending through saidshield plate into the ends of said tubes.

6. A shell and tube heat exchanger comprising a shell, means for feedinga first fluid through said shell, a number of heat exchange tubesextending within said shell, tube plates between which said heatexchange tubes extend and which extend across said shell so as to definewith said shell inlet and outlet and boxes for fluid passing throughsaid tubes and an intermediate heat exchange region through which saidtubes extend, means for feeding a second fluid to said inlet end box andfrom said outlet end box so that it passes through said tubes in heatexchange with said first fluid, a fluid-cooled shield plate spaced infront of the tube plate defining said inlet end box in the direction offiow of said second fluid to shield that tube plate from hot inletsecond fluid, means for fluid cooling said shield plate, flow meansextending through said shield plate for the passage of said second fluidfrom said inlet end box to said tubes, and means for flow communicationbetween the opposite faces of said shield plate to prevent substantiallydifferential pressure across said plate.

7. A heat exchanger according to claim 6 further comprising a layer ofrefractory material in said inlet end box to protect said shell fromsaid second fluid.

8. A heat exchanger according to claim 6 in which said shield platecomprises a pair of spaced plates defining between themselves a chamber,and an inlet and an outlet from said chamber for the passage of coolingfluid through said chamber to cool said plate.

9. A heat exchanger according to claim 8 in which baflies are providedin said chamber.

10. A heat exchanger according to claim 6 in which said flow meansthrough said shield plate comprises stub tubes extending through saidshield plate into the ends of said tubes.

11. A heat exchanger according to claim 10 in which said stub tubes andsaid tubes are in non-sealing engagement, whereby substantialdifferential pressure is prevented across said shield plate.

12. In a shell and tube heat exchanger comprising a shell, a bank oftubes within said shell, and tube plates between which said tubesextend, the provision of a fluid-cooled shield plate spaced from, andadjacent to, one of said tube plates for shielding that tube plate fromvery hot inlet fluid to be passed through said tubes, means for passinga cooling fluid through said shield plate, and flow communication meansfor ensuring that opposite faces of said shield plate are kept atsubstantially identical pressures, whereby said tube plate can besubjected to a differential pressure but is shielded by said shieldplate from large differential temperatures, and said cooled shield platecan be subjected to said very hot inlet fluid and is not subjected toany substantial differential pressure.

References Cited UNITED STATES PATENTS 2,615,688 10/1952 Brumbaugh -134X 2,986,454 5/1961 Jewett 23288 3,132,691 5/1964 Esleeck 165-158 X3,185,210 5/1965 Kuhne et al. 165-134 MEYER PERLIN, Primary Examiner A.W. DAVIS, JR., Assistant Examiner US. Cl. X.R. 165-158, 173

